Device to determine the thickness of a conductive layer

ABSTRACT

The device comprises at least a measuring head ( 7 ) with a transmitter ( 7 A) and a receiver ( 7 B). The head comprises means to measure the thickness through surface resistivity and optical means to measure said thickness by measuring the transparency of the substrate and of the relative layer applied to it.

TECHNICAL FIELD

The present invention relates to a device for contact free measurementof the thickness of a layer of material, particularly conductivematerial, such as a metal or the like, deposited with a vacuumdeposition procedure on a substrate or medium, such as a sheet of paper,a plastic film or the like.

STATE OF THE ART

One of the most urgent requirements in the plastic film and papermetallization industry is the need to determine the quality of thetreatment in terms of uniformity of the coating, that is the layerdeposited. In the case of films which are transparent and have uniformcoloring, the most commonly used method to determine this coating is bymeasuring the transparency. In fact, the thicker the coating, the moreopaque the treated material will be. The sizes of interest are therefore“transmittance” and “optical density”. The equipment normally used iscomposed of a system of photometer heads. The photometers can be used inthe case of treatment with metallic coatings on transparent anduniformly colored films as they work in the visible or in the infrared.

With regard to metallization on paper or on film that does not haveuniform coloring, such as a preprinted film, however, it is not possibleto use an optical system to detect the thickness of the coating. In thiscase the trend of the surface electric resistance may be used as anindicative parameter of the quality of metallization, as the thicker thecoating, the lower the surface resistance is. The parameter of referenceis also the surface resistivity of the film in some specificapplications, such as films for the capacitor industry.

The method for contact free measuring of surface resistivity is to makeuse of the attenuation which a radiofrequency electromagnetic field,emitted by an emitter, undergoes while passing through the film or othersubstrate or medium. The lower the surface resistance and, consequently,the thicker the coating, the greater the attenuation is. Thisattenuation depends on the intensity of the current induced in themetallized layer, said current causing a dispersion in power. Thesurface resistance may be determined in two different ways:

-   -   by measuring with a receiver, positioned in front of the emitter        on the opposite side in relation to the substrate, the field        attenuated by the metallized medium. Examples of devices which        operate with this method are described in U.S. Pat. No.        4,220,915;    -   by measuring the power dispersed due to eddy currents in the        metallized layer. Examples of devices based on this technology        are described in IT-B-1229313, GB-B-1452417; GB-B-1108084.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to produce a measuring devicewhich allows the thickness of the coating to be measured on a pluralityof substrates or media.

In substance, according to the invention a device is provided, forcontact free measurement of the thickness of a thin layer coated on asubstrate, comprising at least a measuring head with a transmitter and areceiver, in which said head comprises means to measure said thicknessthrough the surface resistivity and means to measure said thicknessthrough optical measuring of the transparency of the medium and of therelative layer applied to it.

It is thus possible, with the same head or with a plurality of headssubstantially equivalent to and side by side with one another, toperform measurements both by transparency and based on the surfaceresistivity of the metallized substrate, according to the type ofmaterial produced. According to a possible embodiment, the measurementsaccording to the two techniques are alternate, in the sense that acentral control unit will, on the basis of programming by the operator,activate either one or the other of said two measurement systems,according to requirements and in particular to the nature of themetallized substrate. However, the possibility of using the twomeasurement methods simultaneously is not excluded. According to yetanother embodiment, the two measurement systems may in any case alwaysbe active and the control unit may visualize or in any case alternatelyuse the measurements obtained with one or other of the two techniques.

According to a practical and preferred embodiment of the invention, thetransmitter comprises a transmission coil, to produce an electromagneticfield, and an optical emitter positioned coaxially to said transmissioncoil, and the receiver comprises a receiver coil, to detect theelectromagnetic field emitted by the transmission coil and an opticalreceiver positioned coaxially to the receiver coil, the substratepassing between said transmission coil and said receiver coil.

In this manner a particularly compact device and a head of modest sizeare obtained, in which the space inside the coils, which constitute theinductances forming the transmitting and receiving antennae to produceand receive the electromagnetic field, is exploited to house opticalemission and receiving means.

In a practical embodiment the transmission coil and the receiving coilare each wound on a respective bobbin, made of plastic or in any casenon-ferromagnetic material, inside which the optical emitter and theoptical receiver are respectively positioned.

Further advantageous features and embodiments of the invention areindicated in the attached dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall now be better understood according to thedescription and attached drawing, which shows a non-limiting practicalembodiment of the invention. More specifically, the drawing shows:

FIG. 1 a diagram of vacuum metallization system, with a measurementdevice according to the invention;

FIG. 2 a side view and partial schematic section of a measuring head;

FIG. 3 a partial cross-section and front view according to the lineIII-III in FIG. 2;

FIG. 4 a circuit diagram of the part of the optical emitter and of therelative electronic components;

FIG. 5 a circuit diagram of the part of the optical receiver and of therelative electronic components;

FIG. 6 a circuit diagram of the part which produces the electromagneticfield to measure surface resistivity; and

FIG. 7 a circuit diagram of the part receiving the electromagneticfield.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 generically indicates a system for vacuum metallization of acontinuous substrate, for example a plastic film or a paper strip,indicated with F. The numeral 1 generically indicates the vacuum chamberor bell jar, inside which vaporization sources 3 are positioned. Thenumeral 5 indicates a process roller, on which the film F to bemetallized is driven, delivered from a bobbin B1 and which, after havingbeen metallized, is rewound on a bobbin B2.

Positioned along the path of the film F are various driving rollers andthe heads to measure the thickness of metallization, indicated as awhole with 7, are arranged in an appropriate position. FIG. 1 shows asingle head, it being understood that the others are alignedorthogonally to the plane of the figure, so as to cover the entiretransverse extension of the film. The heads may also be arrangedstaggered in relation to one another in the direction of advance of thefilm F, to provide a greater number of measuring heads and hence obtaina more accurate measurement.

As can be seen in FIG. 2, each head 7 has two parts indicated with 7Aand 7B which respectively house the optical and electromagnetic emissionelements and the optical and electromagnetic receiving elements. In thediagram in FIG. 2 the numerals 9, 10 and 11, 12 schematically indicatethe electronic boards which house the circuit part of the head.Connected to the electronic board 10 is an inductance formed of a coilor solenoid 13 wound in an annular seat of a bobbin 17 mounted on theelectronic board 10. The inductance formed by the coil 13 is inserted inan electronic circuit which shall be described below with reference toFIG. 6.

The bobbin 17 has an axial through hole 17F which ends with a conicalflared part 17C, facing the part 7B of the head. Housed inside the axialhole 17F is a piano-convex focusing lens 18 and positioned behind thisis a light source 19 (not shown in FIG. 2), carried by the board 10 andconstituted by a solid state laser with integral photodiode. This isfitted in a circuit which shall be described below with reference toFIG. 4.

Furthermore, the bobbin 17 has two longitudinal grooves diametricallyopposite to clamp the electronic board 9 in position.

Electrically connected to the electronic board 11 of the part 7B of thehead 7 is an inductance constituted by a second coil or solenoid 23,housed in an annular seat formed in a bobbin 27 fixed to the electronicboard 12 and substantially equivalent to the bobbin 17. In the samemanner as this, the bobbin 27 is also provided with a through hole 27Fending with a conical flare 27C facing the part 7A of the head. Theinductance formed of the coil 23 is fitted in an electronic circuitwhich shall be described with reference to FIG. 7.

Housed inside the hole 27F is a plano-convex convergent lens 28positioned in front of a photosensitive element 29, for example aphotodiode, fixed on the board 12 and which receives radiation emittedfrom the source 19 which manages to pass through the film F with therelative metallization coating. The photosensitive element 29 isinserted in a circuit which shall be described in detail with referenceto FIG. 5.

Analogously to the bobbin 17, the bobbin 27 also has two diametricallyopposite longitudinal grooves in which the board 11 is inserted.

With reference to FIG. 4, the solid state laser 19 forming the source iscontrolled by a circuit, indicated as a whole with 31, which comprisesan oscillator 33 and a programmable gain amplifier controlled by amicroprocessor 37. The signal of the oscillator 33 is amplified by theamplifier 35 and then transferred to a control buffer with automaticgain control, indicated with 39, of the laser 19. The circuit 31 alsocomprises a monitor photodiode 41 which continuatively reads the laser19 emission and the signal of which is amplified by an amplifier 43. Theoutput signal of the amplifier 43 is filtered through a band pass filter45 with a band centered on the frequency of the emission pulses of thelaser 19. This laser, in fact, is controlled in pulsed mode, for exampleat a frequency around 1 kHz, so as to obtain high peak powers respectingthe dissipation limits of the device.

The use of the pulsed laser also makes it possible to reduce the effectsof ambient luminosity and any offsets. In fact, if the photodiode 29 isnot saturated the light pulses emitted by the laser 19 can be easilydiscriminated by the background and, moreover, the offsets and darkcurrent of the photodiode 29 (FIG. 5) can be compensated.

The output signal from the band pass filter is sent to a peak detector47 and the output signal from this is compared in a comparator 49 with areference signal 51. The output signal from the comparator 49,appropriately amplified by an amplifier 53, is used to regulate the gainof the automatic gain amplifier 39 of the laser 19. The emission poweris varied using the amplifier 35 and the change in emission isimplemented when the signal received is excessively low or excessivelyhigh. In the first case emission is increased and in the second case itis decreased.

The circuit which contains the photodiode or other photosensitiveelement 29, generically indicated with 55 and represented schematicallyin FIG. 5, comprises a programmable gain amplifier 57, a band passfilter 59, centered on the frequency of the pulses of the laser 19, apeak detector 61 and a microprocessor 63. The microprocessor 63 may varythe gain of the amplifier 57 if necessary in the event of the signalreceived being too low or too high.

The microprocessors 37 of the circuit 31 of the various measuring headsarranged in sequence along the transverse direction of the film F areconnected to one another by a serial line, as are the processors 63 ofthe circuit 55. The serial line connects the processors of variousmeasuring heads to a central control unit, not shown. This is used tocontrol the heads and to acquire the results of the reading. Inparticular, it is used to set the type of reading (optical or bydetecting the surface resistance) to be implemented as a function of thetype of substrate to which the coating is applied. Alternatively, theheads may constantly operate according to both types of reading and thecontrol unit may be programmed to acquire and process information comingfrom only one of the operating modes, possibly displaying it or storingit in a suitable manner. Again alternatively, the central unit may beprogrammed to process both types of measurement simultaneously.

The photodiode 29 receives an optical signal inversely proportional tothe thickness of the coating on the film F and this signal is used as aparameter to measure the thickness of the coating. Variations in thesignal during transit of the film F in front of the measuring head arean index of an oscillation in the thickness of the coating.

Operation of the optical measuring means described above in the entireinterval of optical density required means that the dynamic range of thesignal is very wide. For example, an optical density variable from 0 to4 implies a dynamic range of 1 to 10,000. This is very difficult toproduce. The method used to overcome this difficulty consists individing the operating range into several intervals with a scale changesystem. The signal received by the photodiode 29 is compared with twothresholds preset by the microprocessor 63. If the resulting value isabove the high level, to avoid saturation of the receiver, amplificationof the system must be reduced, while if it is below the lower thresholdamplification must be increased to obtain a significant signal.Variation of total amplification is obtained both by acting on theamplifier 57 on the receiving side and on the power emitted, through theamplifier 35. Indeed, if the signal received is very low, comparablewith the background noise, amplification of the signal received throughthe amplifier 57 on its own has no advantages. In this case it isnecessary to increase the signal-to-noise ratio. This is obtained byincreasing the signal inciding on the photodiode 29, that is byincreasing the power emitted.

The control circuit of the electromagnetic field emission inductance 13is schematically represented in FIG. 6 and indicated as a whole with 71.It comprises a radiofrequency generator 73, a band pass filter with highselectivity 75, ceramic or SAW, which receives the input signal producedby the generator 73 and the output of which is amplified by an automaticgain amplifier 77. The energizing output signal from the latter isapplied to the coil or inductance 13. An emission control circuit isalso provided to maintain electromagnetic emission of the inductance 13constant.

The control circuit comprises a measuring circuit 79 of energization ofthe transmitter which produces a voltage signal proportional to thesignal emitted by the inductance 13. This is compared in a comparator 81with a signal of reference 83. The output of the comparator 81, suitablyamplified by an amplifier 85, controls the gain of the amplifier 77.

Moreover, the energizing signal is also processed by the microprocessor37, described with reference to FIG. 4.

Positioned on the part 7B of the head 7 is a receiving circuitgenerically indicated with 91, represented schematically in FIG. 7, toreceive the electromagnetic signal which, emitted by the inductance 13,passes through the metallized film or substrate F and reaches theinductance 23. The circuit 91 comprises a differential amplifier 93,which receives the incoming signal picked up by the inductance 23 andthe output of which is connected to a band pass filter 95 with highselectivity, ceramic or SAW, the pass band of which, just as that of theband pass filter 75 of the circuit 71, is centered on the emissionfrequency of the radiofrequency generator 73. The pass signal passesthrough a diode 97 and a mean square value detector, indicated with 99.The signal thus obtained is sent to the microprocessor 63, alreadydescribed with reference to FIG. 5.

It is understood that the drawing merely shows a practical example ofthe invention, which may vary in form and layout without howeverdeparting from the scope of the underlying concept of the invention.

1. A device for contact free measurement of the thickness of a thinlayer coated on a substrate, the device comprising: a measuring headincluding optical means for measuring the thickness of the thin layercoated on the substrate, said optical means measuring the transparencyof the substrate and of the relative layer applied to the substrate,said optical means including an optical transmitter and an opticalreceiver, said optical transmitter and said optical receiver beingarranged on opposite sides of said substrate, said optical transmitterand said optical receiver measuring attenuation of an optical signalpassing through the substrate, said head comprising surface resistivitymeans for measuring the thickness of the thin layer coated on thesubstrate through surface resistivity, said surface resistivity meanscomprising a transmitter with a transmission coil and a receiver with areceiver coil, said transmitter producing an electromagnetic field, saidreceiver detecting the electromagnetic field emitted by saidtransmission coil, said receiver and said transmitter measuringattenuation of said electromagnetic field when electromagnetic fieldpasses through the substrate, said transmitter coil and said receivercoil being arranged on opposite sides of said substrate, the substratepassing between said transmission coil and said receiving coil, saidoptical transmitter being arranged coaxially with said transmissioncoil, said optical receiver being coaxially arranged with said receivingcoil.
 2. A device in accordance with claim 1, wherein said transmissioncoil is wound on a bobbin, inside which said optical emitter ispositioned.
 3. A device in accordance with claim 1, wherein saidreceiver coil is wound on a bobbin, inside which said optical receiveris positioned.
 4. A device in accordance with claim 2, wherein saidoptical emitter comprises a light source and a collimation lens.
 5. Adevice in accordance with claim 3, wherein said optical receivercomprises a focusing lens and an optical sensor.
 6. A device inaccordance with claim 1, further comprising a plurality of said headspositioned side by side.
 7. A device in accordance with claim 1, furthercomprising a main control unit, with which said at least one measuringhead is interfaced.
 8. A device in accordance with claim 7, wherein aplurality of measuring heads are provided, said heads being connected tosaid main control unit through a serial line.
 9. A device in accordancewith claim 7, wherein both means to measure the thickness throughsurface resistivity and optical means to measure the thickness bymeasuring the transparency of the substrate are simultaneously active ineach measuring head, and wherein said control unit only processes thedata coining from one or other of said measuring means.
 10. A device inaccordance with claim 7, wherein both means to measure the thicknessthrough surface resistivity and optical means to measure the thicknessby measuring the transparency are simultaneously active in eachmeasuring head, and wherein said control unit processes the data coiningfrom one or other of said measuring means for the same substrate.
 11. Adevice in accordance with claim 7, wherein said main control unitalternately activates the measuring means through surface resistivity orthe optical means in each measuring head.
 12. A device for contact freemeasurement of the thickness of a layer coated on a substrate, thedevice comprising: a measuring head having a first part and a secondpart, said first part being disposed opposite said second part such thatsaid first part and said second part define a space, the substrate beinglocated within said space; an optical means having an opticaltransmitter arranged in said first part and an optical receiver locatedin said second part, said optical transmitter emitting light directed atthe substrate, said substrate being exposed to said light, said opticalreceiver detecting attenuation of light after the substrate is exposedto said light, said optical means measuring the transparency of thesubstrate and the thickness of the layer applied to the substrate viasaid attenuation of light; a surface resistivity means having atransmitter located within said first part and a receiver arranged insaid second part, said transmitter having a transmission coil, saidtransmitter producing an electromagnetic field, the substrate beingexposed to said electromagnetic field, said transmission coil beingarranged coaxially with said optical transmitter, said receiver having areceiver coil, said receiver detecting attenuation of saidelectromagnetic field, said receiving coil being coaxially arranged withsaid optical receiver, said surface resistivity means determiningsurface resistivity of the substrate from said attenuation of saidelectromagnetic field, said surface resistivity means determining thethickness of the layer coated on the substrate via said surfaceresistivity.
 13. A device in accordance with claim 1, wherein: saidtransmission coil is wound on a first support, said first support havinga first support central seat, said optical emitter being positionedinside said first support central seat; said receiver coil is wound on asupport, said support having a support central seat, said opticalreceiver being positioned inside said support central seat; a collimatorlens is arranged in front of said optical transmitter, said collimatorlens frontally closing said first support central seat; and a focusinglens is arranged in front of said optical receiver, said focusing lensfrontally closing said support central seat.